In-flight upset - 154 km west of Learmonth, WA, 7 October 2008,

such as liquids or small loose fragments of metal that can cause short circuitsbetween components on a circuit board as they move around.Hardware faults can be reproduced in testing, either because they are a hard fault(such as a broken wire) or are a soft fault that can be reproduced usingenvironmental influences (such as temperature extremes, see section 3.6.4).Extensive visual examination and functional testing was conducted on ADIRUs4167 and 4122 and on all of the modules from each ADIRU without any relevantproblems being identified (Appendix E).A more complex fault might involve a component that has a characteristic thatvaries from its nominal value for some reason, such as age, environmental and otherfactors. For example, a resistor might exhibit a change in its resistance as it ages, oras the ambient temperature changes. These variations can change a component’sbehaviour in such a way that it is no longer completely compatible with othercomponents, resulting in the reduced reliability of their interactions. These types offaults can only be triggered by a specific set of operational and/or environmentalconditions, and the system will operate normally until these conditions exist.In summary, although some aspect of the affected units’ hardware was probablyassociated with the failure mode (section 3.5.5), it was very unlikely that ahardware fault triggered the failure mode. A much more likely scenario was that amarginal hardware weakness of some form made the units susceptible to the effectsof some type of environmental factor, which triggered the failure mode.3.6.4 Physical environment factorsThe properties of electronic components are susceptible to change as a result ofvarious mechanisms, including:• temperature extremes and rapid temperature changes• vibration• contamination by liquids or particles (such as dust)• chemical changes (including corrosion)• mechanical stress• electromagnetic interference (discussed in section 3.6.5)• single event effects (discussed in section 3.6.6)These factors may cause a temporary change (such as when an integrated circuitstops operating due to excessive temperature) or a permanent change (such as whenthat temperature is high enough to cause physical damage). Over time,combinations of these factors can lead to degradation of the electronic componentsthrough processes such as fatigue and corrosion.The LTN-101 was designed to meet the requirements of various specifications(including DO-160C 143 and Airbus specifications), and units were tested during143DO-160C (Environmental Conditions and Test Procedures for Airborne Equipment) was anindustry standard produced by the RTCA for aircraft equipment manufacturers. It covered a rangeof different types of environmental effects on equipment (see also Appendix E).- 137 -